This invention relates to an apparatus for detecting registration error, and more particularly to an apparatus of a color imaging apparatus or color display apparatus for fast detecting displacement among a plurality of video signal components such as red, blue or green signals which constitute a video signal generated during the usual imaging or displaying operation, in a "registration error" for registration correction of a color television camera, or convergence correction of a color image pickup tube, or the like.
In a color television camera, which divides a color signal representing an object into three primary color signals (red (R), green (F) and blue (B) signals) and picks up the primary color signals by using a plurality of image pickup tubes, a registration correction function is necessary for superimposing all the color signals precisely.
Even in a color television camera with a computer-based automatic registration correction system, conventional registration correction is carried out after aging of the camera for a long period and after the circuit has settled in a stable condition. This is because a registration state immediately after power is turned on varies due to thermal drift of a deflection circuit and thermal deformation of lens and prism, as the time elapses.
In a conventional registration error detection method a chart is used. Registration error is detected in advance of operation of the camera, and a correction is performed on assumption that the registration state does not fluctuate during the operation. However, even after a sufficient aging, there arise: (1) a variation of terrestrial magnetism depending on the directivity of the camera, and (2) a variation of magnification chromatic aberration of its lens caused by change in a zoom ratio. Therefore, an amount of registration error which has been detected initially does not necessarily adequately correspond to the operating state. On this account, it is not possible to keep the best registration state invariably.
In order to overcome the above-mentioned problems, there is disclosed a method of detecting registration during the operation and correcting the registration immediately upon the occurrence of a registration error, in Japanese Patent Publication (JP-A-62-285590). The conventional technology is based on calculation of correlation between the R or B signal and the G signal, and detection of a shift value associated with the maximum correlation value for evaluating the registration error.
A usual object to be imaged have colors, and registration error is not necessarily detected accurately due to influence of signals of the colors. Based on this reason, the colors of the object are detected and the detection result of the registration error for a picture with abundant colors is excluded, thereby to prevent increasing registration error caused by the detection error.
In the above-mentioned conventional technology, a video signal constituting a television image is coarsely sampled and the sampling density is increased by means of interpolation of the signal. Then the correlation function of the interpolated video signal is calculated, and the interpolation is repeated, thereby to detect the registration error at intended accuracy. The accuracy of interpolation operation directly influences the detection accuracy of the registration error, and it is necessary to use a multiplier with many bits for dealing with the intricate operation. As mentioned above, although the technique of the patent publication features to use a smaller number of sample points for detection of the registration error, it is not necessarily suitable for a fast operation, since the interpolating operation is used. Moreover, a multiplier is used for calculation of the correlation, in digital signal processing, resulting in an increased scale of circuitry and higher cost.
The method of discriminating legitimacy of a signal generated upon the detection of the registration error described in the above-mentioned conventional technology is applied only to objects with abundant colors, and other causes of erroneous detection include noises, out-focusing, etc. Therefore, sufficient reliability cannot be attained through the color-based judgement alone.